Detection and imaging out to the near infrared (wavelengths greater than 940 nm) have been a weak point of standard image intensifier and night vision systems. Standard night vision systems using Gen II (S-20, S-25) and Gen III (GaAs NEA) based photocathodes have little or no photosensitivity beyond wavelengths of 940 nm. However, sensitivity beyond those wavelengths is desirable. Night sky radiation begins to increase dramatically beyond 950 nm wavelength and most existing detectors and imagers cannot observe this increased night sky irradiance. More importantly, most standard photocathode systems cannot detect or utilize the active imaging capability of near infrared based lasers such as the Nd:YAG laser with 1.064 .mu.m monochromatic radiation.
Recently, non-field assisted transmission mode photocathodes capable of imaging Nd:YAG laser radiation have been developed by Varo, Inc. These devices take advantage of an active layer of indium-gallium-arsenide coupled with an aluminum-gallium-arsenide window layer. Although these devices have sensitivity to near infrared wavelengths, increased sensitivity would be desirable for some applications.
Moreover, it would be desirable to have a photocathode with photosensitivity in the near infrared range of 1 to 1.7 .mu.m. In particular, certain designators and laser rangefinders used in military applications employ erbium-doped glass lasers which produce radiation with a wavelength of between 1.4 and 1.5 .mu.m. Existing image intensifiers and night vision systems are not capable of detecting radiation with wavelengths in this range.
In photocathodes with an indium-galium-arsenide active layer, detecting longer wavelengths requires the indium-gallium-arsenide active layer to have a high percentage of indium. For example, it is believed that a photocathode capable of detecting radiation from an erbium-doped glass laser would require an indium percentage in the active layer between forty and sixty percent. As the percentage of indium increases, however, crystal stress increases due to variations between the lattice constant of the active layer and the lattice constant of the window layer.
When the indium concentration reaches such a high level, degradation in crystal quality is significant. Increased crystal stress due to the mismatch of the lattice constants between the window and active layers causes both light scatter and electron scatter. Irregularities in the lattice due to crystal stress cause light scatter. In an irregular lattice, photons are not absorbed properly, but instead are reflected and scattered within the crystal. Lattice irregularities also prevent electrons from easily escaping the lattice and instead tend to cause electrons to scatter. These effects combine to sufficiently lower the quantum efficiency of such a device so as to make it unacceptable for a standard image intensifier in a night vision system.